Hello everyone, I am working on understanding how Kirchhoff's Radiation Law applies in the real world. Basically, the absorbed solar radiation must equal the thermal radiation if a surface is to be at equilibrium. Certain relationships follow from this assumption, namely, for an opaque material, absorptance is equal to emissivity coefficient. Here is the problem I am having. If I look up radiation properties for different materials, I can find an absorptance coefficient and an emissivity coefficient and they are not equal. Why does this occur? This seems to contradict Kirchhoff's law! I have included a couple of links for an example of what I am talking about. http://www.solarmirror.com/fom/fom-serve/cache/43.html https://books.google.com/books?id=X...nepage&q=absorptance emissivity table&f=false Can anyone resolve this contradiction for me?